1. Field of the Invention
The invention relates to the application of multiple polymeric coatings onto a fibrous substrate. More particularly, the invention pertains to the application of multiple different polymeric coatings onto fibrous substrates without regard to chemical or physical incompatibilities of the polymeric coating materials.
2. Description of the Related Art
Developments in the art of fiber-based composites, and particularly in the art of ballistic resistant articles, are being driven by the need for higher performance, increased functionality and lower costs. As materials become more sophisticated and complex, processing techniques do as well. For example, in the ballistic resistant articles industry, ballistic resistant composites are formed high strength fibers which fibers may be encapsulated or embedded in a polymeric binder (polymeric matrix) material to form woven or non-woven fabrics that are either rigid or flexible. These composites display varying degrees of resistance to penetration by high speed impact from projectiles such as bullets, shells, shrapnel and the like.
It is known that the rigidity, impact and ballistic properties of the articles formed from such ballistic resistant composites are affected by the tensile modulus of the binder polymers coating the fibers. For example, U.S. Pat. No. 4,623,574 discloses that fiber reinforced composites constructed with elastomeric matrices having tensile moduli less than about 6000 psi (41,300 kPa) have superior ballistic properties compared both to composites constructed with higher modulus polymers, and also compared to the same fiber structure without one or more coatings of a polymeric binder material. However, low tensile modulus polymeric binder polymers also yield lower rigidity composites. In certain applications, particularly those where a composite must function in both anti-ballistic and structural modes, there is needed a superior combination of ballistic resistance and rigidity. Further, in addition to these properties, it may be desirable for ballistic resistant fabrics to exhibit good environmental resistance, particularly resistance to dissolution, penetration and/or transpiration by potentially harmful liquids, such as sea water and organic solvents. Unfortunately, few conventional binder materials are capable of providing all the desired properties discussed herein. However, it has been found that the combination of multiple layers of different polymeric materials may combine the beneficial properties of each individual material into a single fabric structure.
In the art, two general coating techniques are used to coat polymeric binder materials onto fibers or fabrics. In one method, a series of fibers are coated or impregnated and then formed into a web. In another method, fibers are first formed into a web or fabric and then coated. However, in some situations, neither method is acceptable for the application of multiple coatings of different materials, particularly when the coatings comprise materials that are incompatible. Accordingly, there is a need in the art to enhance the ability to discreetly apply materials that are physically or chemically incompatible onto fibrous substrates, and to reduce the number of individual steps needed to do so.
The present invention provides a new technique for introducing multiple materials into a fibrous web in one process wherein a polymeric material is transferred from a support to a fibrous substrate that has been pre-coated with another polymeric material. In a preferred embodiment, process of the invention is a hybrid of a specific pan-fed reverse roll coating technique and a particular dip and squeeze technique. Composites fabricated from used each of these coating techniques separately have achieved varying degrees of success. A pan-fed reverse roll coating technique is desirable because there is very tight control of the amount of resin matrix binder that is added to the product. However, applying a liquid binder material onto a dry fibrous web causes spread fibers to constrict into discreet yarn bundles to varying degrees, depending on the yarn characteristics. A dip and squeeze technique is desirable because a liquid binder material is introduced to the fibers before yarn spreading, thereby enhancing the spreading of yarn bundles. However, differences in viscosity, line speed, nip pressure, etc., can cause substantial fluctuations in the amount of liquid picked up by the fibrous web. Additionally, in the formation of unidirectional tapes (“UDT”), which are well known in the art, the UDT has very little strength in the transverse direction, even though it is very strong in the machine direction due to the fiber orientation. Accordingly, the attachment of a release paper (or other support) gives the UDT some transverse strength until it can be formed into a fabric. However, with the dip and squeeze coating technique, the bond of a UDT to the support can be inconsistent and weak and may hinder further processing into a fabric. The proposed hybrid combination of these two methods overcomes the problems associated with the two techniques individually.
Importantly, the process of the invention allows for the application of different polymeric binder materials onto a fibrous substrate to take advantage of the beneficial properties of the different materials. These materials can be applied to the fibers without regard to chemical or physical incompatibilities, and this technique allows for combinations of materials where step-wise introduction would otherwise not be possible. The process is also suitable for applying multiple layers of the same polymeric material onto a substrate.